Method for inspecting and measuring sample and scanning electron microscope

ABSTRACT

As an aspect for realizing accurate observation, inspection, or measurement of the contact hole with large aspect ratio, a method and a device to scan a second electron beam after scanning a first electron beam to a sample to charge the sample are proposed wherein the beam diameter of the first electron beam is made larger than the beam diameter of the second electron beam.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of application Ser. No. 12/238,171 filedSep. 25, 2008 now U.S. Pat. No. 7,960,696.

BACKGROUND OF THE INVENTION

The present invention relates to a scanning electron microscope forinspecting and measuring a sample by scanning an electron beam to thesample, particularly to a method and a scanning electron microscope forinspecting and measuring a sample by scanning an electron beam to asample, charging the sample, and scanning another electron beam to thesample under a charged state.

In recent years, in the light of high integration and micronization of asemiconductor element, a variety of patterns are formed on a sample (asemiconductor wafer, for example), and evaluation and measurement oftheir shape and size are becoming important more and more.

Particularly, in a contact hole for obtaining electric conductionbetween layers in multilayering, the diameter of the hole is becomingminute as micronization proceeds, and a contact hole with an aspectratio (depth of contact hole/diameter of hole) of over 30 is notuncommon at present.

To observe and measure this contact hole, it is necessary to detect asecondary electron excited by a primary electron beam (hereinafterreferred to also as electron beam), however, as the aspect ratio becomeslarger, the possibility that the secondary electron collides the sidewall of the hole and disappears in the hole becomes high, and as aresult, there is a problem that observation and measurement of thebottom of the hole become difficult. To solve this problem, thesecondary electron generated at the bottom of the hole is required to betaken out to outside of the contact hole at least.

To realize this, there is a technology (hereafter referred to also aspre-dose) that, by performing preliminary irradiation of a primaryelectron beam prior to electron beam scanning for inspection andmeasurement (hereinafter referred to also merely as observation), theregion for inspection and measurement is made charge positively therebytaking out of the electron from the contact hole is facilitated.

In the patent document 1, a technology is described wherein, prior toelectron beam scanning for observation, an electron beam is irradiatedto wider area than that by the magnification of the time of observation(that is, by a lower magnification than that for observation) includingthe observation region, thereby pre-dose is realized.

Also, in the patent document 2, a technology is described wherein, priorto electron beam scanning for observation, a sample is subjected topreliminary irradiation of an electron beam whose secondary electronemission efficiency δ is larger than 1.0, the surface of the sample ischarged positively, thereafter an electron beam whose secondary electronemission efficiency δ is nearer to 1.0 compared with the electron beamused for the preliminary irradiation is scanned, thereby sampleobservation is performed while maintaining the positively charged statestably.

Further, the patent document 3 describes that, in the relation betweenthe magnification in preliminary irradiation and the positive chargevoltage, the larger the area of preliminary irradiation, the higher thepositive charge voltage which is formed on the sample.

Also, in the patent document 4, a method is described wherein, anelectron emission means called flat gun, other than an electron opticalsystem for observation and measurement, is additionally provided withinthe microscopy body of an electron microscope and charge is formed byoverall irradiation by a large electric current.

-   (Patent Document 1) Japanese Published Unexamined Patent Application    No. H5-151927 (corresponding to the U.S. Pat. No. 5,412,209)-   (Patent Document 2) Japanese Published Unexamined Patent Application    No. 2000-200579 (corresponding to the U.S. Pat. No. 6,635,873)-   (Patent Document 3) WO03/007330 (corresponding to the U.S. Pat. No.    6,946,656)-   (Patent Document 4) Japanese Published Unexamined Patent Application    No. 2000-208579 (corresponding to the U.S. Pat. No. 6,232,787)

SUMMARY OF THE INVENTION

In recent years, because of further micronization of semiconductordevices, contact holes with large aspect ratio are formed, and it isnecessary to form higher charge than before for its observation andmeasurement, which in turn requires to secure a larger irradiationregion of the electron beam for pre-dose than before. However, if theirradiation region is enlarged, there is a problem that it takessubstantially long time before the charge is stabilized.

Existence of the region not irradiated by the beam between scanninglines is considered to be its reason. Movement of the electric chargebetween the position the beam for pre-dose was irradiated and theposition not irradiated and accompanying variation of charge quantitywithin the irradiated region are considered to be the reasons of thevariation of the charge. If the number of scanning lines remainsunchanged, the larger the irradiated region of an electron beam, thelarger the non-irradiated region, and its harmful effect becomesconspicuous.

Also, lowering of electron density of the primary electron beam when theirradiated region is enlarged may possibly be the reason. Charge isdetermined by the continuous balance between the incident quantity ofthe electron beam and the emission quantity of the secondary electronfrom the sample, and stability of charge means the state wherein thisbalance becomes 1.0 practically. Accordingly, as the electron density ofthe electron beam lowers, the time required until the charge isstabilized inevitably becomes longer.

Further, because the electric potential is different between the centerof pre-dose region and the peripheral part and the potential gradient isformed, drift of the electron beam may possibly occur.

In the patent documents 1, 2 and 3, measures against such problems arenot taken at all.

Also, as is described in the patent document 4, it might be possible tomake the surface of the sample be charged by overall irradiation by aflat gun, the flat gun can only irradiate to the irradiation regionwhich covers considerably larger area than the field of view (FOV) of anelectron microscope, therefore it might be possible that the chargeaffects even to the region not related to observation and measurement.

Moreover, if electrons are to be supplied selectively to a limitedregion including FOV, an optical element such as a lens or an alignerequivalent to an electron microscope becomes required. Further, becausethe flat gun is required to be fixed so that the electrons are suppliedfrom a direction different from the light axis of the electron beam(here, out of the orbit where the electron beam of the electronmicroscope can pass) due to a physical restriction, there is also aproblem that maintaining uniformity of charge within the irradiationregion at a high level becomes difficult.

With respect to the charge formed on the sample, it is necessary tomaintain intra-face uniformity of the charge highly precisely and theregion of observation and the center of the charge should coincide,however maintaining intra-face uniformity highly precisely is difficultbecause of the configuration that electrons are supplied from adirection different from that of the light axis of an electron beam.

Further, if an optical element different from an optical system of anelectron microscope is to be arranged aiming highly precise irradiationby the flat gun, it becomes a noise source against the electron beam ofthe electron microscope, therefore it becomes a cause of the shift ofthe position of the beam for observation and measurement and increase ofthe beam diameter, and becomes a factor inhibiting high resolution orhighly precise measurement.

Below, a method and a device for realizing highly precise observation,inspection or measurement of a contact hole with large aspect ratio willbe described.

As one aspect for achieving the purpose described above, in a method anda device wherein, after a sample is charged by scanning a first electronbeam to the sample, a second electron beam is scanned for observation,inspection or measurement of the sample, a method and a device isproposed wherein the beam diameter of the first electron beam is madelarger than the beam diameter of the second electron beam.

According to such technique and configuration, pre-dose is performedusing an electron beam with a beam diameter larger than that of thesecond electron beam, therefore variation of charge originated toexistence of the region not irradiated by a beam can be inhibited. Thesecond electron beam is the electron beam for inspection andmeasurement, and it should be extremely narrowed to improve the spatialresolution of the device. On the other hand, if the electron beam isnarrowed in pre-dose, the region not irradiated by a beam is enlarged asdescribed above. Accordingly, by enlarging the beam diameter of theelectron beam in pre-dose compared with the beam for observation and thelike, the region not irradiated by a beam described above can be reducedor eliminated, which enables inhibiting of variation of charge inpre-dose.

Also, as another aspect for achieving the purpose described above, in amethod and a device wherein, after a sample is charged by scanning anelectron beam to the sample, another electron beam is scanned forobservation and the like, a method and a device is proposed wherein, incharging the sample, firstly, an electron beam whose secondary electrongeneration efficiency δ1 is larger than 1.0 is scanned to the regionhaving a first magnitude, and secondly, another electron beam whosesecondary electron generation efficiency δ2 is smaller than said δ1 isscanned to a region narrower than the first magnitude, thereafter anelectron beam is scanned and inspection or measurement of the sample isperformed.

According to such configuration, observation and the like becomespossible under uniformed charge condition.

Further, as further other aspect applying the pre-dose technology, byshiftingly forming the charge region with respect to the observationregion, oblique irradiation of an electron beam becomes possible.

According to one aspect described above, quick stabilizing of the chargein pre-dose becomes possible, and as a result, realization ofspeeding-up of observation and the like becomes possible. Also,according to another aspect, described above, realization of highaccuracy measurement based on uniformed charge within the electron beamscanning region becomes possible. Further, according to further otheraspect described above applying the pre-dose technology, realization ofobservation by an oblique beam becomes possible while inhibiting anaberration formed by a lens.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the overall configuration of a scanningelectron microscope.

FIG. 2 is a drawing showing shape of a contact hole formed on a waferand the relation between the orbit of a secondary electron generatedtherefrom and the surface charge.

FIG. 3 is a drawing showing the relation between an electron source, afirst condenser lens and a diaphragm, and an electron beam.

FIG. 4 is a drawing showing a technique for increasing the quantity ofan electron beam by changing the focal point of a first condenser lensin performing pre-dose.

FIG. 5 is a drawing showing a technique for increasing the quantity ofan electron beam by changing the diameter of an aperture of a diaphragmon a passage of the electron beam in performing pre-dose.

FIG. 6 is a drawing showing the relation between the irradiation regionof an electron beam on a wafer and the pixel of a device indicating asecondary electron image.

FIG. 7 is a drawing showing the relation between the scanning intervalof a primary electron beam in a high observation magnification and thehalf band width of a primary electron beam.

FIG. 8 is a drawing showing a scanning interval of a primary electronbeam in such a low observation magnification as in the case pre-dose isperformed and a half band width of the primary electron beam.

FIG. 9 is a drawing showing a process flow of the case pre-dose isperformed using an electron beam whose half band width is made largerthan the scanning interval by changing the focus of an objective lens.

FIG. 10 is a drawing schematically showing the relation between themagnetic field of an objective lens and the potential distribution inthe vicinity of a wafer applying to a secondary electron generated fromthe wafer.

FIG. 11 is a drawing showing the relation of the open angle of anelectron beam irradiated onto a wafer, the aberration of an objectivelens, and the half band width of the electron beam calculated therefrom.

FIG. 12 is a drawing schematically showing the potential distribution inthe vicinity of a wafer generated by the charge formed on the surface ofthe wafer by pre-dose.

FIG. 13 is a drawing showing the result of a simulation by a computer onthe potential distribution formed on the surface of a wafer by pre-doseand the potential distribution when the second stage pre-dose isperformed.

FIG. 14 is a drawing schematically showing the potential distributionformed by pre-dose and the orbit of the primary electron beam accordingto the position of incidence.

FIG. 15 is a drawing schematically showing the secondary electron imageof a contact hole generated when the contact hole is observed by anelectron beam entering a wafer perpendicularly or obliquely.

FIG. 16 is a drawing schematically showing the position relation betweenthe charge region formed by pre-dose and the observation region, andlooks of a secondary electron image obtained thereby.

FIG. 17 is a drawing showing the overall configuration of a scanningelectron microscope equipped with an energy filter for measuring charge.

FIG. 18 is a drawing schematically showing the difference of the orbitaccording to the voltage of an energy filter and energy of a secondaryelectron.

FIG. 19 is a drawing schematically showing the relation between thetotal quantity of secondary electrons passing the energy filter andcharge of the sample.

FIG. 20 is a drawing schematically showing the difference of the orbitof a primary electron beam according to a magnitude relation of energyof a primary electron beam and the potential on the surface of a wafer.

FIG. 21 is a drawing schematically showing the difference of the orbitof a primary electron beam according to a magnitude relation of thecharge on the surface of a wafer, with respect to the wafer voltage andthe electron quantity detected by a secondary electron detector.

FIG. 22 is a drawing schematically showing a sample of a display onto ascreen of the database relating the wafer kind and the pattern size withthe pre-dose condition.

FIG. 23 is a drawing showing a process flow for setting an optimalpre-dose condition referring to the database and actually determiningwhether the pre-dose condition is good or not using a wafer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the description below, mainly in pre-dose, acceleration voltage ischanged to the same with higher secondary electron generationefficiency, and a half band width of an electron beam irradiatedinterlocking with the area of the preliminary irradiation region ischanged. Thus, high charge voltage can be formed on a wafer and theelectron beam is irradiated to an entire area of the irradiation region,therefore provision of a method and a device capable of forming thecharge effectively becomes possible.

Also, a method and a device is described wherein, preliminaryirradiation is performed by different acceleration voltage afterformation of charge, leveling of the charge formed on the surface of awafer is performed, thereby leveling of the electric field in thevicinity of the wafer formed by the charge voltage becomes possible, andthe electric field component in the direction perpendicular to the lightaxis of the electron beam can be decreased. Thus, even if the potentialon the surface of the wafer changes with the lapse of time, the electronbeam is not subjected to deflection by the electric field component inthe lateral direction, therefore occurrence of drift is inhibited.

Furthermore, because the pre-dose technology described below uses anelectron optical system used for observation, structural modificationsuch as a flat gun is not required. Therefore, cost increase can beinhibited, and constitution of the device satisfying requirement ofusers is possible.

Below, the embodiments of the pre-dose technology will be describedreferring to drawings.

FIG. 1 shows the overall configuration of a scanning electronmicroscope. A whole control unit 43 controls the whole device through anelectron optical system control device 44 and a stage control device 45on the basis of the acceleration voltage of an electron, the informationof a wafer 21, the observation position information and the like, inputby an operator from a user interface 42.

The wafer 21 is fixed onto a sample stage 25 in a sample chamber 26through a wafer conveying device not shown after going through a sampleexchange chamber.

In accordance with the direction from the whole control unit 43, theelectron optical system control device 44 controls a high voltagecontrol device 31, a retarding control unit 39, a first condenser lenscontrol unit 32, a second condenser lens control unit 33, a secondaryelectron signal amplifier 34, an alignment control unit 35, a deflectionsignal control unit 40, and an objective lens control unit 37.

A primary electron beam 13 drawn out from an electron source 11 by adraw out electrode 12 is converged by a first condenser lens 14, asecond condenser lens 16 and an objective lens 20 and is irradiated ontothe wafer 21. On the way, the electron beam is subjected to adjustmentof its orbit by an alignment coil 18 and is two-dimensionally scanned onthe wafer 21 by a deflection coil 19 which received a signal from thedeflection signal control unit through a deflection signal amplifier 36.To the wafer 21, retarding voltage (negative voltage in an electronmicroscope) is applied from the retarding control unit 39 to slow downthe electron beam. Originating irradiation of the primary electron beam13 to the wafer 21, a secondary electron 24 discharged from the wafer 21is captured by a secondary electron detector 17 and is used as abrightness signal of a secondary electron image display device 41through the secondary electron signal amplifier 34. Also, because thedeflection signal of the secondary electron image display device 41 andthe deflection signal of the deflection coil 19 are synchronized, thepattern shape on the wafer 21 is faithfully reproduced on the secondaryelectron image display device 41.

Further, a diaphragm 15 is arranged on the irradiation light axis of theprimary electron beam 13. This diaphragm 15 has a function of adjustingthe quantity of the primary electron beam irradiated to the wafer 21 andhas an action of producing a beam open angle to minimize the totalquantity of the aberration (diffraction aberration, chromaticaberration, spherical aberration) of the objective lens 20.

The electron optical system control device 44 is constituted to controlthe negative voltage applied to the sample, the deflection coil(scanning deflector) and the lens according to the established energyreaching the sample, the magnification, the beam diameter and the likeof the electron beam. For example, switching of electron beam energyreaching the sample is performed by adjustment of the applied voltage ofnegative voltage to the sample. Also, switching of energy reaching thesample may be performed by changing the applied voltage to, for example,an accelerating electrode (not shown) or an electrode equivalent to it.

For inspecting and observing the pattern on the wafer 21 at high speed,it is necessary to detect the height of the wafer 21 when the samplestage 25 moves to the desired observation point, and to match the focusof the objective lens according to the height. Therefore, wafer heightdetecting function using light is provided. The position of the samplestage is detected by a sample stage position detect unit 38, a heightdetect laser beam emitter 22 irradiate the light toward the wafer 21since the sample stage 25 nears the vicinity of a predeterminedposition, its reflection light is received by a position sensor 23, andthe height of the wafer 21 is detected based on the light receivingposition.

And a focus amount according to the detected height is fed back to theobjective lens through the objective lens control unit 37. As a result,when the sample stage 25 reaches a predetermined position, the focus hasbeen already set, and detection of the pattern can be performedautomatically without operation by an operator. Further, althoughdescription is made below referring to an example of a semiconductorwafer as an observation object by an electron beam, there is nonecessity to limit to it and a mask or the like used in transcription ofa semiconductor pattern may be its observation object.

The objects of measurement using a scanning electron microscope forsemiconductor processes are gate wires, wire width for bit calling of atransistor, and the aperture diameter of the contact hole for securingelectrical conductivity between layers. With respect to the contacthole, in particular, as multilayering proceeds accompanying highintegration of a semiconductor, the depth of the contact hole isdeepened, and the aperture diameter becomes small accompanyingmicronization.

For example, in some of the 45 nm node semiconductors which currentlyare under development for commercialization, the depth of the contacthole is 1.5 to 2.0 μm and the aperture diameter is approximately 50 nm.In such a contact hole, the aspect ratio, which represents the ratio ofthe depth of the hole to the aperture diameter, exceeds 30.

FIG. 2 shows an example of the shape of a contact hole and an orbit of asecondary electron. Generally, the contact hole 52 is provided forconnecting between layers of a semiconductor device. Therefore, asubstrate 51 is constituted of conducting material such as silicone, aninsulation film 50 is formed thereon, and the insulation film 50 isetched to form the contact hole. In the drawing, H represents the depthof a contact hole 52, D represents the diameter of an aperture at thebottom of the contact hole 52, and the aspect ratio of such contact hole52 is represented by H/D. To measure the aperture diameter D of thecontact hole 52, not only the secondary electron from the hole surfacebut also the secondary electron 24 from the bottom must be detected.However, in a contact hole with a large aspect ratio, the secondaryelectron 24 generated at the aperture part of the hole, in many cases,collides the side wall of the hole and disappears on the way oftravelling upwardly from the hole, and cannot be captured by thesecondary electron detector 17. As a result, the shape of the aperturepart cannot be constructed as a secondary electron image, therefore theaperture diameter cannot be measured as well.

As a means for solving this problem, a technique called pre-dose to formcharge on the wafer 21 is adopted. Pre-dose means a method wherein anelectron beam is irradiated to the surface of the wafer 21 in which thecontact hole is shaped to produce positive charge 53, and the secondaryelectron is lifted up to the surface of the wafer 21 by an electricfield of the potential difference against the bottom of the contacthole.

In the gazette for the Japanese Published Unexamined Patent ApplicationNo. 2000-200579, for example, with respect to the acceleration voltageof a primary electron beam irradiated to produce charge, it is disclosedthat the secondary electron generation efficiency (the ratio ofgeneration of the secondary electrons from the wafer 21 to the incidentquantity of the primary electron beam to the wafer 21) must be 1 orabove. By performing this pre-dose, easy detection of the secondaryelectrons generated from the bottom of the contact hole 52 becamepossible, and observation of the shape and measurement of the size havebeen performed.

However, there are problems also in this pre-dose method. As describedabove, to form the positive charge on the surface of the wafer 21, it isnecessary to irradiate a primary electron beam for a fixed period oftime and form the stable charge. Also, to lift up the secondaryelectron, a definite electric field is required, and this electric fieldis determined by the surface potential of the wafer 21 and the depth Hof the contact hole. When the contact hole is deep, higher positivecharge is required compared with the case of a shallow one. As isdescribed also in WO03/007330, the irradiation area of the primaryelectron beam should be made wide to raise the charge voltage. However,as the irradiation area is made large, the density of the electronwithin the irradiation region is lowered, therefore the time until thecharge is stabilized becomes necessary. As a result, because measurementtakes longer time, the throughput is lowered and the cost is increased.Below, an embodiment effective in forming charge on the surface of thewafer 21 efficiently will be described.

FIG. 3 shows the relation of the electron emitted from the electronsource 11, and the first condenser lens 14 for converging it and thediaphragm 15 disposed on the passage of the electron beam. Onto thediaphragm 15, the electron 13, which is emitted from the electron source11 and converged at a position apart from the diaphragm 15 by a distanceL by a magnetic field 60 formed by exciting the first condenser lens 14,is irradiated. A round-shape aperture with a diameter D is formed in thediaphragm 15, and a portion of the electrons 13 reached to the diaphragmpasses this aperture and reaches onto the wafer 21.

The ratio of the electrons 13 which reach the diaphragm 15 to theelectrons 61 which are a part thereof and pass the diaphragm 15 is samewith the ratio of the solid angle of each of them. Consequently, inperforming pre-dose, if the ratio of this solid angle is changed, thequantity of the electrons reaching the wafer 21 can be increased,decrease of the electric charge density by irradiation of electrons to awide area can be inhibited, and the time required for pre-dose can beshortened. FIG. 4 shows a technique of changing the electron quantity bychanging the convergence position of the first condenser lens 14. When Lrepresents the distance between the convergence position of the firstcondenser lens 14 and the diaphragm, and D represents the diameter ofthe diaphragm, the approximation equation for the solid angle θ of theelectron beam passing the diaphragm can be represented by the equation(1).θ=(D/L)²  (1)

A current amount passing the diaphragm is proportionate to this solidangle. For example, as shown in FIG. 4, the crossover position inobservation and measurement of the shape is P1, the distance to thediaphragm becomes L1, and a little current is used to decrease thedamage of the sample. On the other hand, when pre-dose is performed, alarge current is required, therefore the crossover position is changedto P2 and the distance to the diaphragm is shortened to L2. If thecrossover position P2 is changed to, for example, such a position assatisfying the equation of 2L2=L1, the current amount passing thediaphragm becomes 4 times based on the equation (1). If this relation isapplied, the irradiation electron quantity in pre-dose can be changedoptionally.

FIG. 5 shows a technique to change the electron quantity by changing theaperture diameter of the diaphragm. In the diaphragm 15, a plurality ofapertures with the different diameter are provided. The aperture usedfor measurement is with a smaller diameter of D1 for letting a littleelectric current passes, and the aperture used in pre-dose is with alarger diameter of D2 for letting a large electric current passes. Theratio of the electron quantity when these different apertures are usedfollows the equation (1), and, for example, when 2D1=D2, the electronquantity becomes 4 times.

So far, a method to form charge efficiently by increasing the electronquantity in pre-dose has been described. Apart from it, by the relationbetween the scanning interval of the electron on the wafer 21 and thehalf band width of the electron converged by the objective lens also,the time required for charging and the charge voltage differ as well.FIG. 6 shows the irradiation region on the wafer 21 and the scanninginterval of the electron beam. LH and LV in the drawing represent thelength of the irradiation area 62 of the primary electron on the wafer21, and are determined by the magnitude of the display region of thesecondary electron image display device 41 and the observationmagnification in displaying on the display device. For example, when thedisplay region of the image on the image display device 41 is 135 mm×135mm and the observation magnification is 1,000 times, LH and LV becomethe same length which is 135 mm÷1,000=0.135 mm. Further, the displaydevice has pixels of a finite number, and the shape of the pixelsgenerally is square whose numbers m and n become equal.

When the number of the pixel is 512, the interval of the scanning lineson the wafer 21 can be calculated by the scanning area on the wafer 21and the pixel number, and becomes 0.135 mm÷512 pixels≈264 nm. FIG. 7shows the relation between the scanning interval on the wafer 21 and thehalf band width of the electron beam in observation by a highmagnification. The resolution of electron beam devices used forsemiconductor processes is approximately 2 nm at present. Even at a lowacceleration voltage (for example, the secondary electron generationefficiency is said to be highest when the acceleration voltage isapproximately 300 V to 400 V in a silicon dioxide-based insulation filmused for semiconductor) with high secondary electron generationefficiency (ratio of the generated quantity of the secondary electronsto the quantity entering the wafer 21 of the primary electron beam) usedfor pre-dose, the resolution is approximately 4 nm. When observed by ahigh magnification of, for example, 200,000 times, the scanning intervalof the electron beam on the wafer 21 becomes 1.32 nm from the relationdescribed above.

Consequently, in such observation by a high magnification, as shown inFIG. 7, because the scanning interval is smaller than the half bandwidth 63 of the electron beam, the electron beam is irradiated to theentire scanning area on the wafer 21. However, as shown in FIG. 8, inthe scanning area used in performing pre-dose, the scanning interval ofthe electron beam, which is, for example, approximately 264 nm in thecondition of 1,000 times described above, is much larger than the halfband width 63 of the electron beam, which is 4 nm, for example,therefore if such electron beam with small half band width is used, theelectron beam is not irradiated to major part of the irradiation region.In such a case, because of the effect of the region where the electronbeam is not irradiated, charge formed on the surface of the wafer 21becomes low in average.

Accordingly, it is possible that the charge required for observation ofthe contact hole with high aspect ratio cannot be formed. To solve sucha problem, pre-dose is carried out in accordance with the proceduresshown in FIG. 9. First, move to a measuring point where observation by ahigh magnification is performed (S11). Thereafter, the magnification andthe acceleration voltage are set to the execution condition of pre-dose(S12), then, the exciting current of the objective lens is set to eitherstronger exciting side or weaker exciting side than the exciting currentintrinsically required for convergence on the wafer 21 by theacceleration voltage for pre-dose (S13). After setting, pre-dose isperformed (S14).

After pre-dose is finished, the condition of the electron optical systemis set to the magnification, acceleration voltage, and exciting currentused for observation (S15), and observation by a high magnification isexecuted (S16). With respect to the condition of the exciting current inS13, it is important that the half band width of the electron beam onthe wafer 21 is set to at least larger than the scanning interval. Also,in this example, a half band width is used for exemplary description asan indicator representing the beam diameter, but it is not necessary tolimit to it, and if there is a more appropriate parameter forrepresenting the magnitude of the beam diameter, such a parameter may bedefined for the beam diameter. In pursuing the aim of leveling theunevenness of the potential distribution within the electron beamscanning region in pre-dose, if there is a more suitable parameter, itis desirable to select it.

First, a scanning interval is calculated in accordance with a controlprogram in the electron optical system control device 44 based on theacceleration voltage and the magnitude of the irradiation region inpre-dose. Based on the calculated scanning interval, the excitingcurrent of the objective lens for realizing the electron beam having alarge half band width larger than at least the scanning interval iscalculated in accordance with the exciting current control program.Also, it is desirable that the electron beam diameter required isrealized by setting the exciting current to the stronger exciting side.The reason will be described referring to FIG. 10. On the wafer 21, amagnetic field 70 of the objective lens 20 is leaked out and a magneticfield distribution 71 is formed on a light axis 72 of the electron beam.Further, to the secondary electron 24 generated by irradiation of theelectron beam 13, a force F shown in the equation (2) is applied.F=e(v×B)+e·E  (2)where: e represents energy of the secondary electron. The first term ofthe equation (2) represents the force by the magnetic field 70 appliedto the secondary electron, v is the kinetic vector of the secondaryelectron 24, and B is the vector representing the direction of themagnetic field of the objective lens.

Also, the second term of the equation (2) represents a force by theelectric field applied to the second electron, and E is the vectorrepresenting the line of electric force in the vicinity of the wafer 21determined by the voltage applied to the wafer 21 by the retardingcontrol unit 39 and the potential of the objective lens, or the chargevoltage of the wafer 21 accompanying generation of the secondaryelectron 24.

When the positive charge is formed in the wafer 21, the force to drawback the secondary electron to the wafer 21 becomes strong according tothe second term of the equation (2) resulting in neutralization of theformed charge, and the charge voltage lowers. Under such situation, ifthe magnetic field 70 is strengthened, the first term of the equation(2) becomes large which results in hindering the secondary electrongenerated from the wafer 21 from returning to the wafer 21, and largepositive charge can be formed on the wafer 21. This is the reason why itis desirable to set the exciting current in pre-dose to the strongerexcitation side and to enlarge the electron beam diameter.

On the other hand, forming of excessive charge in the wafer 21 leads toincrease the risk of electrostatic breakage at the boundary face of thecontact hole, therefore forming appropriate charge is important. Eventhough the charge voltage of the wafer 21 lowers because of the reasondescribed above if the weaker exciting side is used, there is also anadvantage of lowering the risk of electrostatic breakage. Accordingly,the electron beam diameter may be enlarged in the weaker exciting side.

FIG. 11 shows a method using aberration of the objective lens forenlarging the half band width of the electron beam. The half band widthR of the electron beam can be represented by the equation (3).

$\begin{matrix}{R = {\left( {0.61 \times {\lambda/\alpha}} \right)^{2} + \left( {\left( {1/2} \right) \times \left( {\Delta\;{E/E}} \right) \times C_{c} \times \alpha} \right)^{2} + \left( {\left( {1/4} \right) \times C_{s} \times \alpha^{3}} \right)^{2} + \left( {R_{ss} \times M} \right)^{2}}} & (3)\end{matrix}$Here, the first term represents a blur due to diffraction of anelectron, λ represents the wavelength of the electron beam, and αrepresents the open angle (half angle) of the electron beam on the wafer21. The second term represents a blur due to the chromatic aberration,ΔE represents the expanse of energy the electron beam has, E representsthe acceleration voltage of the electron beam in pre-dose, C_(c)represents the chromatic aberration coefficient of the objective lens.The third term represents a blur due to the spherical aberration, andC_(s) represents the spherical aberration coefficient of the objectivelens. The fourth term represents reflection of the magnitude of theelectron source onto the wafer 21, R_(ss) represents the magnitude ofthe electron source, and M represents the magnification of the electronoptical system. The graph shown in FIG. 11 shows the relation of theopen angle, various aberrations and half band width of the electronbeam. As will be understood from the graph, if the open angle isdecreased and diffraction aberration is enlarged, or if the open angleis enlarged and spherical aberration is enlarged, the half band width ofthe electron beam can be enlarged. In the case of using diffractionaberration out of them, the open angle for attaining the half band widthof 264 nm becomes approximately 0.14 mrad.

This open angle is, if an open angle in usual observation with highresolution is provisionally assumed to be approximately 14 mrad,approximately 1/100 of it, and it is obvious that the beam currentcannot be secured. If this open angle is to be achieved by an electronoptical system, the aperture diameter of the diaphragm 15 is to beexemplarily made 1/100 of the usual aperture diameter (0.2 μm againstthe usual aperture diameter of 20 μm, for example), however, consideringhow the aperture of this dimension is prepared with high accuracy orpossible contamination of the aperture part by irradiation of anelectron beam, it is obvious that stable operation as an apparatus forindustrial use is difficult.

Accordingly, the method of enlarging the half band width of an electronbeam by enlarging the spherical aberration of an objective lens isconsidered to be appropriate. For example, in a scanning electronmicroscope with the spherical aberration coefficient of 0.4 mm, toachieve the half band width of 264 nm by spherical aberration, the openangle may made approximately 138 mrad in accordance with the equation(3). If the method shown in FIG. 4, for example, is to be applied, thismeans the distance between the first condenser lens and the diaphragm inobservation is to be made 1/10 which is technically possible.

Also, if the method shown in FIG. 5 is to be applied, the aperturediameter of the diaphragm may be made 10 times, therefore this method istechnically possible as well. Further, it is obvious that, by jointlyapplying this method of enlarging the half band width of an electronbeam by enlarging spherical aberration with the method shown in FIG. 4or FIG. 5, increase of the half band width and increase of the beamcurrent can be realized simultaneously.

Moreover, according to the aspect ratio of the contact hole, requiredcharge quantity differs. Because excessive charging increases the riskof dielectric breakdown of the insulation film 50, it is important tocharge appropriately. Although the relation between the irradiation areaof pre-dose and the charge voltage is described in WO03/007330, thecharge voltage can be controlled by adjusting the time for pre-dose.Therefore, it is also important to adjust the time for pre-dose shown inS14 in FIG. 9 according to the aspect ratio of the contact hole shapedon the wafer 21 observed.

Also, so far, the case of inhibiting non-irradiated region of a beam byenlarging the beam diameter in pre-dose has been described, however,because enlarging the beam diameter means decrease of the irradiationamount per unit area, it is preferable to jointly use the method toincrease the electron quantity as described above to supplement it. Forexample, when the magnitude of the beam diameter is controlled to equatewith the distance between scanning lines to fill the space between thescanning lines, the magnitude of the beam diameter changes according tothe magnification. Consequently, by controlling so that the beam currentincreases as the magnification decreases (as the scanning region iswidened), above supplement becomes possible. Furthermore, the absolutequantity of the beam fed in is adjustable by control of the number offrames (the number of times of scanning of two-dimensional region),therefore the beam current may be controlled instead of or jointly withcontrolling of beam current amount.

So far, a method for increasing the electron beam quantity in pre-doseand a method to increase the half band width of the electron beamconsidering the scanning interval on the wafer 21 in pre-dose forefficiently performing pre-dose have been described. Although, pre-doseitself is a very important technology for charging, the primary purposeis to perform stable observation or reproducible measurement ofdimensions of the bottom of a deep contact hole. To attain the purpose,stable scanning of the primary electron beam on the wafer 21 inobservation performed after pre-dose is important.

FIG. 12 schematically shows the potential distribution generated by thecharge formed on the surface of the wafer 21 by pre-dose. The charge onthe surface of the wafer 21 starts to be formed soon after start ofirradiation of the primary electron beam 13 and the charge voltagegradually increases. As the charge voltage increases, an equipotentialline 73 shown in FIG. 12 extends to the irradiation region and forms apotential black point 74.

The equipotential line 73 extended and the potential black point 74become a potential barrier for the secondary electron generated from thewafer 21 and act as the force to prevent the secondary electron fromtraveling upward and to draw it back to the wafer 21. Also, this barrierdoes not work evenly over the entire irradiation region but worksconspicuously toward the peripheral part within the irradiation region.These phenomena occur continuously within the irradiation region, andafter some time, irradiation of the primary electron 13 and generationof the secondary electron 24 are balanced resulting in formation of thecharge having distribution on the wafer 21. Further, this potentialdistribution varies according to, for example, scanning of the electronbeam in observation and movement of a charge within the wafer 21.

In particular, when the primary electron beam 13 is scanned on the wafer21 for observation, symmetry of charge varies according to itsacceleration voltage and scanning direction. If the symmetry is lost,the potential distribution having the component perpendicular to thetraveling direction of the electron beam and varying with respect totime is formed. As a result, a time change of the reaching position ofthe electron beam onto the wafer 21, that is, a drift phenomenon of theelectron beam, occurs which becomes the cause of a blur of an image inobserving by a high magnification. To solve this problem, it isimportant that the potential distribution generated by pre-dose has notsteep variation, and stabilization of charge voltage by stepwisepre-dose under a plurality of conditions as measures for its realizationwill be described hereinbelow.

FIG. 13 is the result of the calculation by computer simulation of thepotential distribution generated by pre-dose, and both (a) and (b) showthe potential distribution on the center line of the irradiation region.(a) of FIG. 13 is a result of the simulation of the charge potentialgenerated by pre-dose of the first step when 120 μm square on the oxidefilm (silicon dioxide) is made the irradiation region with 300 Vacceleration voltage and 80 pA primary electron beam 21 quantity inpre-dose.

The result shows that the charge potential is highest as approximately180 V in the center of the irradiation region, and has such potentialdistribution as, the farther from the center of the irradiation region,the lower the charge potential. (b) of FIG. 13 shows the result of thesimulation of the charge potential after the pre-dose of the second stepis performed for the irradiation region of 60 μm square with 1,600 Vacceleration voltage and 8 pA primary electron beam 21 quantity afterthe charge potential shown (a) is formed.

It can be seen that the potential distribution of a wide regionincluding the observation point is flattened by pre-dose of the secondstep. The causes of it are that, because the acceleration voltage inpre-dose of the second step is 1,600 V, the generation efficiency of thesecondary electron is inhibited compared to that in 300 V, and that,because a portion of the secondary electrons generated from the pre-doseregion of the second step is attached again to the 60 μm square and itsouter region, the density of the positive charge within the pre-doseregion of the second step lowers thereby the potential lowers as well.

As described above, by dividingly performing pre-dose in at least 2steps, the potential distribution of the observation region at least canbe made uniform or near to uniform state, and inhibiting the driftphenomenon of the electron beam becomes possible. Its principle will bedescribed hereinbelow in more detail.

First, the electron beam with 300 eV energy reaching a sample isirradiated to a non-charged sample and the surface of the sample (theelectron beam with high secondary electron generation efficiency) ischarged (pre-dose of the first step). Because the secondary electrongeneration efficiency in the electron beam of 300 eV is 2.0 under acertain condition, it becomes such a state that 2 nos of electrons areemitted from the sample against 1 no of electron is fed in, and 1 no ofpositive hole is generated in the sample against 1 no of electron is fedin. This in turn forms the positive charge.

Further, this positive charge forms a potential barrier on the sample asdescribed previously. As the potential barrier gradually becomes largerby accumulation of charges, the electrons that cannot go beyond thepotential barrier increase responding to it. Thereafter 1 no of electronis fed in, 2 nos of electrons are emitted from the sample against it,then, under the state that 1 no of electron is returned to the sampleside by the potential barrier, and the state of charge becomes stable.

Next, the pre-dose of the second step is performed by scanning theelectron beam of the reaching energy with low secondary electrongeneration efficiency compared with that of the pre-dose of the firststep. In this example, the pre-dose of the second step is performedusing the electron beam of 1,600 eV reaching energy. The secondaryelectron generation efficiency of the electron beam of 1,600 eV is made1.2. In other words, when 1 no of electron is fed in, 1.2 no of electronis emitted from the sample on the calculation.

With respect to the electron emitted here, 0.6 no of electron returns tothe sample by the potential barrier previously formed. By this, 0.4 noof electron is accumulated in the sample, the charge potential lowers,and the potential barrier becomes slightly small. It becomes the samestate that 1 no of electron is fed in and 1.2 no of electron is emittedfrom the sample. At this time, because the potential barrier is lessthan the initial state, the number of the electrons returning the sampledecreases also (0.35 no, for example). If this state further continues,for example, under the state 1 no of electron is fed in and 1.2 no ofelectron is emitted, it becomes the state that 0.2 no of electronreturns to the sample by the potential barrier. That means the numbersof the electrons fed in and the electrons emitted become equal and thecharge becomes stabilized state.

As described above, by performing the pre-dose of the second step sothat the peak of the potential distribution formed by the pre-dose ofthe first step is flattened, inhibiting of the drift phenomenon and thelike in beam scanning for observation thereafter becomes possible.

In this aspect, the pre-dose of the first step for forming large chargeis performed first, and to shape the potential distribution formed bythe pre-dose of the first step, the pre-dose of the second step isperformed. The condition for this time is the relation of δ1>δ2>1.0between the secondary electron generation efficiency δ1 of the electronbeam used for the pre-dose of the first step and the secondary electrongeneration efficiency δ2 of the electron beam used for the pre-dose ofthe second step. In the state wherein δ2 is less than 1.0 (that is, thestate wherein the electrons fed in is more than the electrons emitted),negative charge continues to increase simply and the charge is notstabilized.

In the pre-dose of the first step, an electron beam having the secondaryelectron generation efficiency that is necessary for that the chargequantity increases first and is stabilized after some time by the effectof the potential barrier formed thereby and the like is used, whereas inthe pre-dose of the second step, an electron beam having the secondaryelectron generation efficiency that is necessary for that the chargequantity decreases first and is stabilized after some time by the effectof the potential barrier formed in the first step and the like is used.

In this aspect, the reaching energy of an electron beam satisfying theconditions described above is set.

Although it was stated as above that reduction of the gradient of thepotential produced by pre-dose is important for reducing drift of theelectron beam, it is obvious that the charge region formed by pre-dosemust include the observation and measurement point. Also, it ispreferable that the center of the irradiation region where pre-dose isperformed coincides with the point where observation and measurement areperformed. One reason is to reduce the drift described above and anotheris for perpendicular incidence of the primary electron beam to the wafer21.

FIG. 14 exhibits the problem occurring when the center of the primaryelectron irradiated for observation is shifted from the center of thecharge region formed by pre-dose. If the primary electron beam 13 ismade incident to the center of the charge region, it is irradiatedperpendicularly to the wafer 21, but if it is made incident from a pointother than the center, it is made incident to the wafer 21 obliquely byrefracting action by an electric field formed by the charge. When thecontact hole 52 is observed using the primary electron beam madeincident perpendicularly, the upper opening and the bottom opening areobserved in the position symmetric to the contact hole 52 as shown inFIG. 15, however, when observed by the primary electron beam 13 madeincident obliquely, the bottom opening of the contact hole 52 is shiftedfrom the center according to the incident direction.

If oblique incidence is extreme with the incident angle larger than theangle of the side wall of the contact hole 52, for example, observationof the edge of the bottom in the incident direction becomes impossibleand accurate shape cannot be captured. Accordingly, in performingpre-dose, alignment for centering the position for observation isimportant using, for example, an alignment coil 18 shown in FIG. 1 andthe like.

On the other hand, by intentionally shifting the point for observationof the contact hole and the like and the center of the charge regionformed by pre-dose, the primary electron beam can be made oblique. Asdescribed above, when pre-dose is performed using the accelerationvoltage wherein the secondary electron generation efficiency exceeds 1,positive charge is formed on the surface of the sample. On the otherhand, the primary electron beam has negative charge. Therefore, theprimary electron beam is inclined by a deflecting action to thedirection where positive charge exists. The inclination angle of theprimary electron beam is proportionate to the charge voltage and thedistance between the charge region and the observation point. FIG. 16 isan example schematically showing the relation of the position ofobservation and the charge region and inclination direction, and thesecondary electron image on the display device.

In FIG. 16 (1), the center of the contact hole for observation and thecenter of the charge region formed by pre-dose coincide, and there is nodeflecting action the primary electron beam receives from the chargeregion in observation in this case. Therefore, the contact hole isobserved symmetrically.

On the other hand, FIG. 16 (2) shows an example wherein the chargeregion is to the left of the center of the observation point. In thiscase, the primary electron beam in observation is subjected todeflecting action of the positive charge formed in the charge region andcurves to the left, and is made incident obliquely to the wafer. As aresult, observation of the left inner wall of the contact hole becomespossible. Although FIG. 16 shows the case wherein the charge region andthe observation point are in entirely different position relation, theobservation point may be included within the charge region.

This inclined observation method does not require new construction forinclination, and is effective as a technique for easy observation of theside wall of a contact hole and a line pattern. Further, becausedeflecting action to the primary electron beam applies in the positionwhere the electron beam has been almost converged, geometric aberrationfor inclination and off-axis chromatic aberration generated by that thebeam passes off the axis of the objective lens are less. Consequently,an inclined image of high resolution can be obtained.

According to one of the aspects described above, even for the contacthole with high aspect ratio formed on the wafer by micronization of asemiconductor, observation and measurement become possible withoutperforming pre-dose for a long period of time.

The technique described above can be used as one of the automaticmeasuring functions normally used in a scanning, electron microscope fora semiconductor. Furthermore, the techniques described in the presentinvention, with the exception of the part that electric current ofpre-dose is changed by enlarging the aperture diameter of the diaphragm,can be realized by changing the control program only with the hardwarestructure of the existing scanning electron microscope remainingunchanged, and adjustment of an electron optical system and the likeaccompanying the modification of the hardware is not required. The usercan enjoy great merits related with introduction such as saving thepurchase cost of functions and shortening the downtime of the device.

As described above, for observation, inspection or measurement of acontact hole with high aspect ratio, it is necessary to form charge on asample by irradiation of an electron beam, however, on the other hand,forming of excessive charge may cause great damage to a semiconductorformed on a wafer. For example, if the thickness of the insulation film50 shown in FIG. 2 is assumed to be 1.5 μm, and the charge voltage afterpre-dose is assumed to be 160 V from the result of FIG. 13 (b), a verystrong electric field of approximately 1.07×10⁸ (V/m) is formed betweenthe surface of the contact hole and the substrate 51.

In the experiments by the present inventors, discharge was not confirmedunder the condition described above, however, if voltage resistingperformance of the insulation film 50 is lower than the above figure,the positive charge accumulated on the surface of the insulation film isdischarged toward the substrate 51, and damage is caused to theinsulation film and the substrate by energy of discharge at that time.

However, the condition under which the discharge phenomenon occursdepends on the kind, thickness, insulation performance, and surfacecharge of the insulation film. Therefore, the irradiation condition ofpre-dose must be determined while confirming possibility of observationof the bottom of the contact hole by performing pre-dose for an actualdevice while preventing occurrence of the discharge phenomenon byformation of excessive charge. At the same time, it is also important tokeep the pre-dose condition and the charge voltage at that time and thelike as quantitative data and lower the risk of occurrence of thedischarge phenomenon in a new device which will be made thereafter.

With this regard, performing pre-dose while measuring charge voltage isproposed as a technique to quantify the condition wherein observation ofthe bottom of the contact hole is possible, and its device constitutionand technique will be described hereinbelow.

FIG. 17 shows a device constitution provided with an energy filter in anelectron optical system as one technique for measuring charge voltage.

An energy filter 80 is a device disposed between the second electrondetector 17 and the wafer 21 to measure the surface charge of the wafer21 by measuring energy of the secondary electron generated from thewafer 21. To the energy filter 80, negative voltage is applied inmeasuring energy through an energy filter control unit 81.

A measuring method of charge voltage by the energy filter 80 will bedescribed briefly referring to FIG. 18. In measuring energy of thesecondary electron 24, negative voltage V_(f) is applied to the energyfilter 80 through the control unit 81. The secondary electron 24 isgenerated from the sample with a variety of initial energy V_(p)according to the difference in the physical generation process.Thereafter, it is accelerated by retarding voltage V_(r) applied to thewafer 21, therefore, immediately before the energy filter, energy V_(e)of the secondary electron 24 becomes V_(p)+V_(r). Further, when thecharge voltage V_(s) by pre-dose is formed on the surface of the wafer21, the energy V_(e) of the secondary electron 24 becomesV_(p)+V_(r)−V_(s). Here, V_(p) and V_(r) are negative potential andV_(s) is positive potential.

When the magnitude relationship of V_(f) and V_(e) is |V_(f)|≦|V_(e)|,the secondary electron 24 can pass through the energy filter because itsenergy is higher than that of the energy filter 80 and can be detectedby the secondary electron detector 17 (FIG. 18 (a)). However, when thevoltage of the energy filter becomes high as |V_(f)|>|V_(e)|, becausethe secondary electron 24 cannot pass through the energy filter, itcannot be detected by the secondary electron detector 17 (FIG. 18 (b)).

Based on this relation, if the secondary electron 24 which has passedthrough the energy filter 80 is detected by the secondary electrondetector 17 while changing the applied voltage V_(f) of the energyfilter 80, S-shaped output curves of the secondary electron detector asshown in FIG. 19 are obtained. In FIG. 19, the curves obtained when theretarding voltage V, applied to the wafer 21 is −2,500 V and the chargevoltage is 0 V and 100 V are exemplarily shown.

When the charge potential V_(s) is 0 V, that is, when the sample is notcharged, such output curve (a) as that quantity of the secondaryelectrons 24 passing through the energy filter and detectable by thesecondary electron detector 17 decreases from where the voltage V_(f)exceeds −2,500 V is obtained, and when the charge potential V_(s) is 100V, such output curve (b) as that quantity of the secondary electrons 24passing through the energy filter and detectable by the secondaryelectron detector 17 decreases from where the voltage V_(f) exceeds−2,400 V is obtained. By catching V_(f) where the detected quantity ofthe secondary electrons 24 decreases, the charge voltage of the samplecan be measured.

FIG. 20 shows a method which utilizes reflection of the primary electronbeam 13 in the vicinity of the sample as another technique for measuringcharge voltage.

The primary electron beam 13 travels within the electron optical systemwith energy V0, is slowed down by the wafer 21 retarding voltage V_(r)to become the desired voltage V_(acc)=V0−V_(r), and is irradiated ontothe sample. Both of V0 and V_(r) are negative voltage, and whenobservation is performed as an electron microscope, the relation oftheir absolute values usually is as |V0|>|V_(r)|, therefore, the primaryelectron beam can reach the wafer 21.

For example, when the surface of the wafer 21 is charged to the voltageV_(s) by irradiation of the electron beam 13, the potential of thesample viewed from the primary electron beam becomes V_(r)+V_(s). Whenthe retarding voltage V_(r) applied to the wafer 21 is −2,500 V and thecharge voltage V_(s) of the sample is 100 V for example, it becomes−2,400 V. Even in this case, if V0 is larger than −2,400 V in thenegative side, the primary electron beam can reach the sample.

However, when the retarding voltage V_(r) applied to the wafer 21 ismade high and the relation with the acceleration voltage V0 of theprimary electron beam 13 becomes |V0|<|V_(r)+V_(s)|, the primaryelectron beam 13 cannot reach the sample but is reflected above thewafer 21 toward the direction of the charged particle source 11 notshown.

As a result of measuring the electron quantity detected by the secondaryelectron detector 17 when the retarding voltage V_(r) is changedutilizing this principle, the output curve shown in FIG. 21 is obtained.The horizontal axis of the graph represents the retarding voltage V_(r),and the vertical axis represents the quantity of the signal of theelectrons detected by the secondary electron detector. Further, theacceleration voltage V0 of the primary electron beam at this time is3,000 V.

Explanation will be given using the case of FIG. 21 (a). (a) representsthe case wherein the surface charge V_(s) of the wafer 21 is 0 V, thatis, the case it is not charged. The region [1] shows the area of thecondition where the primary electron beam is reflected without reachingthe sample because the potential of the sample is higher than the energyof the primary electron beam. At this time, what is detected by thesecondary electron detector 17 not shown is not the secondary electron24 but is the primary electron beam 13.

The region [2] is the region where the primary electron beam 13 canreach the wafer 21 because the energy of the primary electron beam 13 ishigher than the potential of the wafer 21 and can generate the secondaryelectron 24. What is detected by the secondary electron detector 17 isthe secondary electron 24 generated at the wafer 21.

The region [3] is the point where the potential of the sample and theenergy of the primary electron beam are equal and the primary electronbeam is slowed down to 0 V to reach the sample. The primary electronbeam 13 can neither reflect nor generate the secondary electron.Consequently, the signal detection quantity at the secondary electrondetector 17 becomes zero.

FIG. 21 (b) shows the relation between the retarding voltage V_(r) andthe quantity detected by the secondary electron detector 17 when thesurface charge voltage V_(s) of the wafer 21 is V_(sr). By V_(sr), thepotential of the surface of the wafer 21 changes from V_(r) toV_(r)+V_(sr). Therefore, the region [3] shifts by V_(sr) compared with(a).

Using this principle, the retarding voltage wherein the detectionquantity becomes zero is detected by the secondary electron detector 17while the retarding voltage is changed. If the retarding voltage of thattime is V_(rs) and the charge voltage of the sample is V_(sr), thecharge voltage V_(sr) can be measured by the relation ofV0=V_(rs)+V_(sr).

By using the charge measuring method described above, the charge formedon the surface of the wafer by pre-dose can be measured. With respect tocharge measuring techniques, the method using the energy filter and themethod utilizing reflection of a primary electron beam were exhibitedhere, however, it is not necessary to limit to them. For example, atechnique is also possible wherein the charge quantity is calculatedfrom variation of focus current before and after pre-dose of theobjective lens required for making the primary electron beam 13 convergeon the wafer.

Furthermore, by accumulating experiences on optimal pre-dose conditionfor a variety of patterns and making them database, setting of anoptimum condition becomes possible even without measuring charge voltageevery time. FIG. 22 shows an example of the database which is displayedon the device by operation on the user interface 42.

The database includes information related to the wafer such as the nameof the process in manufacturing a semiconductor, the kind and thicknessof the insulation film, dimensions of the contact hole formed, and theconditions of the pre-dose of the first and second steps for it, as wellas the charge voltage amount formed on the wafer 21 by those pre-doseconditions.

By displaying this database on a device in setting the measuringcondition using the device, it becomes possible to find the conditionequal or near to that of the wafer kind on which the measuring conditionis to be set and to incorporate it into the sequence of automaticmeasurement.

Also, in this database, confirmation, addition and deletion of thepre-dose condition responding to the kind of the device are possible.For example, when entirely new type of wafer is measured, its conditioncan be added to this database. On the other hand, the information of theprocess already not in use for production may be deleted according tonecessity.

A setting procedure of a pre-dose condition using the database and themeasurement technique of the charge voltage described above is shown inFIG. 23. First, the wafer to be measured is loaded onto the device(S21). The database is opened through the user interface (S22), andwhether the pre-dose condition corresponding to the Wafer to be measuredalready exists or not is confirmed (S23). If there exists, the pre-dosecondition written in the database can be transferred to the file forautomatic measurement (S24), and if not, the condition of a similarwafer or the condition based on a past result is written in the file forautomatic measurement (S25).

As the first condition setting has been finished, the effect isconfirmed using the set pre-dose condition in an actual wafer and thecharge voltage is measured (S26). Whether observation of the bottom ofthe contact hole according to the set condition was possible or not isconfirmed (S27), and if yes, its device name, pre-dose condition andcharge voltage are added to the database (S28). If not, the pre-dosecondition written in the data for automatic measurement is changed(S29), and the effect is confirmed again by observation of the wafer. Ifthe pre-dose condition can be eventually established, the device name,pre-dose condition and charge voltage are added to the database (S28).

Thereafter, the database is closed (S30), the data for automaticmeasurement with the appropriate pre-dose condition having been writtenis recorded in a recorder not shown (S31), the wafer used is unloadedfrom the device (S32), and setting is finished.

Thereafter, in inspecting and measuring the wafer on which the conditionis decided according to the procedure described above, if this file forautomatic measurement is used, excellent observation can be performedwithout searching for the pre-dose conditions. Further, as it has beenconfirmed already that the damage by discharge would not occur, the riskon the production of the wafer can be avoided.

According to one of the aspects described above, performing ofobservation and measurement of the contact hole with high aspect ratioformed on the wafer accompanying micronization of a semiconductorbecomes possible without performing pre-dose for a long period of time.

The technique described above can be used as one of the automaticmeasuring functions normally used in a scanning electron microscope fora semiconductor. Furthermore, the techniques described in the presentinvention, with the exception of the part that electric current ofpre-dose is changed by enlarging the aperture diameter of the diaphragm,can be realized by changing the control program only with the hardwarestructure of the existing scanning electron microscope remainingunchanged, and adjustment of an electron optical system and the likeaccompanying the modification of the hardware is not required. The usercan enjoy great merits related with introduction such as saving thepurchase cost of functions, shortening the downtime of the device, andreducing the risk of the damage to which the wafer is subjected.

Below, another example of monitoring the charge condition duringpre-dose will be described. As described previously, it is desirable toaccurately set the charge quantity attached by pre-dose. In thisexample, a method for accurately controlling the charge quantity bymonitoring the charge attached by pre-dose in real time will bedescribed.

When positive charge is attached to a sample by pre-dose, the electronemitted from the scanning region is drawn by the positive charge,therefore it is slowed down by that amount. If the initial accelerationvoltage (V_(acc)) of the electron emitted from an electron source is3,000 eV and the retarding voltage (V_(r)) applied to the sample is−2,000 V, the second electron emitted almost without energy (that is, 0eV) is accelerated by an electric field formed between the retardingvoltage and an electrode in the vicinity of the sample and an objectivelens and the like. In the case of acceleration by an electric fieldformed between the sample and the objective lens of a ground potential,the secondary electron is accelerated toward the direction of theelectron source by energy of 2,000 eV.

Under such situation as above, if the desired charge quantity (V_(pre))is +100 V, the acceleration energy of the secondary electron emittedfrom the sample when the desired charge quantity is obtained becomesV_(r)+V_(pre)=1,900 eV (when the secondary electron is almost 0 eV). Inother words, if the energy of the electron emitted from the sampleduring pre-dose can be monitored, catching accurately the timing forstopping scanning for pre-dose becomes possible. If the voltage appliedto the energy filter is set to the vicinity of −1,900 V, the secondaryelectron slowed down by 100 eV or more by pre-dose cannot pass throughthe energy filter. If the relation between the detected electronquantity and the charge quantity of pre-dose is known beforehand, it canbe judged that the time when the detected electron quantity reaches apredetermined value is the time when the desired charge quantity issecured.

Those which can pass through the energy filter with the applied voltageof −1,900 V are only the electrons with the accelerating energyexceeding 1,900 eV. If charging of the sample proceeds by pre-dose, theacceleration energy of the electron emitted from the sample lowersgradually, therefore, the quantity of electrons that can pass throughthe energy filter decreases gradually.

If the control, such that, an electron quantity appropriate for stoppingpre-dose is memorized beforehand relating to the combination of theoptical condition (beam current, landing energy of the electron beam,magnification, and the like) when the applied voltage to the energyfilter is made a predetermined value and the kind of the sample, andpre-dose is stopped when the electron quantity is reached, is performed,same charge condition can be realized stably regardless of the samplecondition which is effective for improving the length measurementreproducibility. Further, measurement of charge is also performed inparallel with pre-dose which is effective for improving throughput.

Although above explanation describes an example wherein pre-dose andcharge measurement proceed simultaneously, it is not necessary to limitto it, and, for example, both of them may be proceeded alternately inseries like pre-dose→charge measurement→pre-dose→charge measurement→ . .. . In this case, pre-dose may be stopped by time control by plottingthe change of the electron quantity with respect to change of time andpredicting the timing when the predetermined electron quantity isreached by extrapolation.

What is claimed is:
 1. A method for inspecting or measuring a samplebased on electrons obtained by scanning a third electron beam to thesample, which has been charged by a first electron beam with a secondaryelectron generation efficiency δ1 greater than 1.0, the methodcomprising: (a) charging the sample by scanning the first electron beamto a first region having a first size; (b) after (a), scanning a secondelectron beam with a secondary electron generation efficiency δ2 lessthan said δ1 to a second region having a second size smaller than thefirst size and contained within the first region; and (c) after (b),scanning the third electron beam for inspection or measurement of thefirst and second regions of the sample.
 2. The method for inspecting ormeasuring a sample as set forth in claim 1, wherein said secondaryelectron generation efficiency δ2 is larger than 1.0 and smaller thansaid δ1.
 3. The method for inspecting or measuring a sample as set forthin claim 1, wherein a center of the scanning of the first electron beamwith said secondary electron generation efficiency δ1 and a center ofthe scanning of the second electron beam with said secondary electrongeneration efficiency δ2 coincide.
 4. The method for inspecting ormeasuring a sample as set forth in claim 1, wherein said secondaryelectron generation efficiency δ2 is set so that a number of electronsentering the sample is greater than a number of electrons emitted fromthe sample, and after some time, both numbers are equalized andstabilized by a potential barrier formed on the sample by the scanningof the first electron beam with said secondary electron generationefficiency δ1.
 5. A scanning electron microscope comprising: an electronsource, a scanning deflector for scanning an electron beam emitted fromthe electron source on a sample, and a control device for adjustingenergy of the electron beam reaching the sample and controlling thescanning deflector, wherein the control device adjusts the reachingenergy of the electron beam and controls the scanning deflector so that,firstly, an electron beam with a secondary electron generationefficiency δ1 larger than 1.0 is scanned to a first region having afirst size, and secondly, another electron beam with a secondaryelectron generation efficiency δ2 less than said δ1 is scanned to asecond region having a second size smaller than the first size andcontained within the first region, and thereafter an electron beam isscanned for inspection or measurement of the first and second regions ofthe sample.
 6. The scanning electron microscope as set forth in claim 5,wherein said secondary electron generation efficiency δ2 is larger than1.0 and smaller than said δ1.
 7. The scanning electron microscope as setforth in claim 5, wherein the control device controls the scanningdeflector so that a center of scanning by an electron beam with saidsecondary electron generation efficiency δ1 and a center of scanning byanother electron beam with said secondary electron generation efficiencyδ2 coincide.
 8. The scanning electron microscope as set forth in claim5, wherein said secondary electron generation efficiency δ2 is set sothat a number of electrons entering the sample is more than a number ofelectrons emitted from the sample, and after some time, both numbers areequalized and stabilized by a potential barrier formed on the sample byscanning of an electron beam with said secondary electron generationefficiency δ1.